Method of forming a chromate conversion coating on magnesium



United States Patent C) IVIETHOD OF FORMING A CHROMATE CONVER- SION COATING N MAGNESIUM Ferdinand P. Heller, Philadelphia, Pa., assignor to Amchem Products, Inc., Ambler, Pa., a corporation of Delaware No Drawing. Filed Mar. 8, 1957, Ser. No. 644,722

'3 Claims. (Cl. 148--6.2)

This invention relates to the art of chemically forming an integral chromate conversion coating on surfaces of magnesium and is particularly concerned with an improved method of maintaining the solution of the bath which is applied to the surface in the treating operation in the best condition for yielding optimum results over extended periods of use.

The term magnesium as used in this specification and in the appended claims is intended to apply not only to relatively pure magnesium but also to alloys thereof in which magnesium is the principal or predominant ingredient.

Before describing the details of the present invention it will be useful to observe that, while magnesium is relatively easy to coat, very few processes heretofore employed for the purpose have proven to be acceptable or successful on a commercial basis. For instance, prior to the present invention some commercial treatments have involved the used of solutions containing dichromate and nitric acid and while magnesium so treated is superior to untreated metal in corrosion resistance yet the results; at best, may be described as fair only.

As another example I may mention the familiar treatment known commercially as the Dow #7 treatment which makes use of a hot solution containing a dichromate and a small amount of calcium fluoride. This process, however, is not without its attendant drawbacks. For instance, in employing this treatment the precleaning cycle is exceedingly important and in order to form a satisfactory conversion coating the time of treatment with the dichromate-calcium fluoride bath is usually in the neighborhood of /2 hour as a minimum. Additionally, since this treatment employs the use of hot solutions, preferably boiling, large amounts of heat are required.

Still another commercial treatment has involved the formation of a fused film of aluminum hydroxide on the magnesium surface, and while this process produces a corrosion resistant surface it is very expensive and time-consuming and results in the production of a brittle film which tends to flake off and leave the base metal unprotected in situations Where the magnesium is subjected to severe mechanical shock or physical distortion.

With the foregoing in mind the principal object of the present invention may be said to reside in the provision of an economical, extremely rapid and easily controlled process by means of which a flexible, corrosion-resistant, chemically-bonded, chromate conversion coating may be produced on magnesium surfaces even over long periods of use of the coating bath or solution. Another object of the invention is to provide a coating of the character described which will serve as an excellent basis for paint, enamel, japan and the like.

How the foregoing together with such other objects and advantages as will appear hereinafter are attained will now be described.

I have discovered that a clean magnesium surface can be coated with an excellent corrosion resistant, paintbonding, chemical conversion film in a very short period of time if the surface is subjected to the action of a coating bath consisting essentially of a solution of hexavalent chromium containing certain very definite amounts of this material plus certain very definite quantities of the anions of mineral acids from the class consisting of hydrochloric acid and nitric acid which baths are further characterized by having a very definite range of acidity as will further appear.

More specifically the solutions used in the process of the present invention must contain from 15 to 25 grams per liter and preferably from 18 to 22 grams per liter of hexavalent chromium expressed as CrO At concentrations less than 15 grams per liter the coatings produced tend to be smutty, soft and non-adherent while at concentrations greater than 25 grams per liter the coatings produced, while possibly decorative in nature, have little or no corrosion resistance, whereas at concentrations between the limits given and especially at concentrations between 18 and 22 grams per liter, extremely tight, fiexible corrosion resistant coatings can be obtained just so long as the other factors necessary to success are also adhered to as will appear below. Incidentally, suitable sources of hexavalent chromium are chromic acid and ammonium, sodium and potassium chromate or dichromate.

In addition to the hexavalent chromium content the bath should contain at least 20 grams per liter of chloride and while apparently there is no upper limit to the quantity of chloride which can be tolerated in the bath I perfer, largely in the interest of economy, not to use more than approximately 160 grams per liter. At concentrations of less than 20 grams per liter the activity of the bath is such that no useful coatings are obtained in any reasonable period of time.

Beyond the foregoing, for each gram of chloride present in the bath there should be present also from 0.8 to 1.5 grams per liter of nitrate expressed as NaNO In general .when the nitrate present is less than 0.8 gram per liter for each gram of chloride present the coatings tend to be soft and non-adherent while at concentrations of nitrate greater than 1.5 grams per liter for each gram of chloride present the coatings tend to lose their corrosion resistance. As a rule, I have found that exceptionally good results can be obtained when the concentration of nitrate is from 0.9 to 1.2 grams per liter for each gram of chloride present.

Furthermore, as indicated, the degree of acidity of the bath is also important. In a bath freshly prepared according to the above teachings the pH will generally be found to lie between 0.1 and 0.7. However, as the bath is used and replenished this pH ceases to be a controlling factor whereas the acidity of the bath should be maintained from 0.4 to 1.0 N to Brom cresol green and preferably from 0.6 to 0.8 N to Brom cresol green. At a normality to Brom cresol green of less than 0.4 the activity of the bath is diminished to such a degree that no useful coatings are formed in short periods of time. At normalities greater than 1.0 N to Brom cresol green the activity of the bath is such that the coatings formed are excessively powdery. By experience I have found that a normality to Brom cresol green of between 0.6 and 0.8 yields optimum results, and for this reason, is preferred practice.

The solution should be used at temperatures between 60 and 100 F. and preferably below F. At temperatures below 60 F. the coatings produced have little corrosion resistance. As the temperature exceeds 90 F. there is developed a marked tendency for the coating to become powdery and at temperatures exceeding F. only relatively useless powdery coatings are normally produced. At the preferred temperature range which 7 lies between 60 and 90 F. I have found that excellent,

corrosion-resistant, powder-free coatings can be consistently produced.

As for the time of treatment-this is not critical because all that is necessary is for the cleaned surface of the magnesium to be subjected to the action of the treating solution for a suflicient length of time to have a visible film produced thereon. In general, when employing an immersion process a treating time of from 30 seconds to 2 min. is sufiicient while in spray processes the treating time may even be less than this.

In carrying out the process of the present invention the magnesium must be clean. This can be accomplished most advantageously by cleaning otf gross soil with a mild alkali cleaner and removing scalev by a suitablepickling agent such as a solution of chromic acid and sodium nitrate. After cleaning any residues of the cleaning fluids used should be removed from the surface by adequate rinsing.

After thorough cleaning as, for example, by the pro cedure just described, the surface of the magnesium is subjected to the action of a coating bath or solution formulated in accordance with the foregoing teachings. For example, a coating bath may be prepared according to the following formula:

Water, to make 1 liter.

In connection with the foregoing formula it should be noted that the bath is formulated from chromic acid, hydrochloric acid and sodium nitrate. However, other ingredients could have been used as a source of hexavalent chromiumfor example, ammonium, sodium or potassium chromate or dichromate could have been used. Furthermore, other suitable sources of chloride ions which could have been employed, are sodium and potassium chlorides. As for the nitrate ions, other suitable sources could have been nitric acid, ammonium nitrate and potassium nitrate. in other words the sources of the several ions are not critical just so long as the proper relationship is maintained.

As the bath is employed to treat either a series of pieces or large areas of magnesium the hexavalent chromium, the chloride and the nitrate, from time to time, should be restored to the desired values. In general this may be done by replenishing the bath, as required, by suitable additions of chromic acid, sodium nitrate and hydrochloric acid in such amounts as will restore the chemical constituents of the bath to their original value and, at the same time, will maintain the free acidity of the bath from 0.4 to 1.0 N when the bath is titrated with standard caustic solution using Brom cresol green as an indicator.

After the coating has been formed the coated object should have adhering traces of the coating solution removed by rinsing with water, and if the object is to be painted, the coating should then be rinsed with a con ventional, dilute, acidulated solution of chromic and/or phosphoric acid. If chromic acid rinsing solutions are used care should be taken not to expose the coated surface to their action for too long a period of time as the freshly prepared coatings at times are moderately soft and may be stripped from the surface by the. action of these rinsing solutions.

The process of the present invention has been evaluated as against other protective treatments for magnesium with the follownig results:

Wrought alloy F.S.l was coated in accordance with the preceding example. Other pieces of F.S.l alloy were treated in approved chrome nitrate solutions and with the Dow #7 process. The alloys were placed on a tidewater exposure for one year on duplicate exposure 1:016. Results after one years exposure are tabulated? elow 4 Rack No. 1

Treatment Rating Process of present invention Slight surface corrosion. Dow #7 Process Panel completely disintegrated. Chrome Nitrate Pickle Do.

Rack

Treatment Rating Slight surface corrosion. Deep pitting corrosion. Panel completely disintegrated.

Process of present invention.-. Dow #7 Chrome Nitrate Pickle Rack N0. 1

Treatment Rating God Process of present invention... 0

Paint failure and eorroswn.

Dow #7 Rack N0. 2

Treatment Rating Process of present invention Good. DOW #7 Paint failure and corrosion.

Similar tests were run on cast alloy H (primed and two coats sky blue lacquer). After one years exposure, results were as tabulated below:

Treatment Rating Pro :css of present invention Perfect. how #7 General blistering.

I claim:

1. In the art of forming a chemical conversion coating on magnesium surfaces wherein the surface is subjected to the action of an aqueous acid solution consisting essentially of hexavalent chromium and the anions of mineral acid from the class which consists of hydrochloric and nitric acids; the method which includes the following steps: preparing a bath of the said solution which contains, per liter, from 15 to 25 grams of hexavalent chromium, from 20 to 160 grams of chloride and from 0.8 to 1.5 grams of nitrate expressed as NaNO for each gram of chloride; heating the bath to from 60 to 100 F.; applying said bath to the surfaces to be coated until a visible film is produced thereon; and replenishing the bath as required, during use, so as to maintain the acidity thereof which is titratable with caus' tie. to the Brom cresol green endpoint, at from 0.4 to 1.0 N.

2. A process according to claim 1 wherein the quantity of hexavalent chromium in the bath is from 18 to 22 grams per liter and the quantity of nitrate is from 0.9 to 1.2 grams per liter for each gram of chloride present, and further, wherein the replenishment is such as to maintain the said acidity at from 0.6 to 0.8 N.

3. The process of claim 2 wherein the temperature of the bath does not exceed F.

References Cited in the file of this patent UNITED STATES PATENTS 2,393,943 Thomas et al Ian. 29, 1946 2,428,749 De Long Oct. 7, 1947 2,760,890 Kosrnos Aug. 28, 1956 64,730 Kinder et al Dec. 16, 1958 FOREIGN PATENTS 584,097 Great Britain Jan. 7, I947 

1. IN THE ART OF FORMING A CHEMICAL CONVERSION COATING ON MAGNESIUM SURFACES WHEREIN THE SURFACE IS SUBJECTED TO THE ACTION OF AN AQUEOUS ACID SOLUTION CONSISTING ESSENTIALLY OF HEXAVALENT CHROMIUM AND THE ANIONS OF MINERAL ACID FROM THE CLASS WHICH CONSISTS OF HYDROCHLORIC AND NITRIC ACIDS, THE METHOD WHICH INCLUDES THE FOLLOWING STEPS, PREPARING A BATH OF THE SAID SOLUTION WHICH CONTAINS, PER LITER, FROM 15 TO 25 GRAMS OF HEXAVALENT CHROMIUM FROM 20 TO 160 GRAMS OF CHLORIDE AND FROM 0.8 TO 1.5 GRAMS OF NITRATE EXPRESSED AS NANO3 FOR EACH GRAM OF CHLORIDE, HAETING THE BATH TO FROM 60* TO 100*F., APPLYING SAID BATH TO THE SURFACES TO BE COATED UNTIL A VISIBLE FILM IS PRODUCED THEREON, AND REPLENISHING THE BATH AS REQUIRED, DURING USE, SO AS TO MAINTAIN THE ACIDITY THEREOF WHICH IS TITRATABLE WITH CAUSTIC TO THE BROM CRESOL GREEN ENDPOINT, AT FROM 0.4 TO 1.0N. 